Optically variable flakes are used in paints and inks for obtaining a color shift between two distinct colors at first and second angles of incident light. Optically variable pigments (OVP) incorporating such flakes play an important role in the security printing industry. By physically aligning optically variable flakes incorporated in an ink or paint, a dramatic and uniform color shift can be created which cannot be reproduced by photocopiers and printers. This desirable feature has led many nations to use optically variable pigments in their currencies.
U.S. Pat. No. 4,434,010 to Ash teaches methods of forming optically variable flakes, inks, paints and devices. An optical coating is formed by vacuum coating a metallized layer on a flexible web which is soluble in a preselected liquid. After the optical coating is formed, the coated web is passed through the preselected liquid and the web is dissolved away from the coating. Depending upon the desired application, the remaining optical coating can be secured on a substrate to form a replicated coating or it may be broken into thin film flakes which are recovered and used in inks and paints. The patent discloses the use of water soluble webs made of polyethylene oxide, methyl-hydroxy-propyl cellulose, and polyvinyl alcohol resin, and webs made of acrylic plastic materials which are soluble in organic solvents such as acetone or methyl ethyl ketone. The methods disclosed for making inks require a flake treatment step to remove residual web material and solution from the produced flakes.
U.S. Pat. Nos. 5,084,351; 5,059,245; 5,135,812; 5,171,363; 5,279,657; 5,281,480; and 5,383,995, each to Phillips, et al. each disclose a method of making optically variable ink wherein a substrate or forming web comprising an insoluble flexible material such as polyethylene terephthalate is coated with a solvent-soluble acrylic polymer known as 517-1, available from Thermark Division of Avery International. The composition of the acrylic polymer is not disclosed. Suitable solvents for dissolving the acrylic polymer are acetone and methyl ethyl ketone. Complete polymer removal with the solvents is not reported, and after filtering, the resultant flakes require a spraying with fresh solvent to remove traces of the soluble polymer.
The use of acrylic polymers as acetone or methyl ethyl ketone soluble web materials has been discussed in U.S. Pat. Nos. 5,059,245 and 5,084,351, both to Phillips, et al. However, it has been found that the specifically suggested polymethyl-methacrylate disclosed in these patents suffer in general from an inability to be even slightly dissolved by acetone in a single pass through the solvent.
Polymeric materials for release coatings should be able to form a uniformly thin coating. The molecular weight and glass transition properties of the polymeric material should render the material stable at the high temperatures associated with vacuum deposition of metal optical layers, without causing defects in the optical layers and in flakes produced from such layers. In addition, preferred polymeric coating materials should be soluble in aqueous solutions and should emulsify into water-based emulsions, and thus not require the use of potentially dangerous solvents to form coating solutions thereof. Furthermore, the polymeric material should form a clear and homogeneous release coating which will not detrimentally affect properties such as clarity and uniformity in topcoated or overcoated metallized optical layers.
The present invention provides the aforementioned desired properties in a release coating material and provides methods of making high-quality, high-clarity topcoated metallized layers having predictable and repeatable properties. The present invention provides a strippable release coating for forming optically variable flakes and devices, which: 1) can be coated on a substrate to form a uniformly thin film layer, 2) is stable under conditions associated with the vacuum deposition of metals, 3) can be formed from a relatively safe, non-flammable aqueous coating emulsion, and 4) can readily and completely dissolve in a single pass through a common, inexpensive and relatively safe organic solvent such as acetone to form metallic flakes from metallized layers coated thereon. Optical, metallic layers of high quality can be formed on the polymeric coating and easily separated from the coating for subsequent use in optically variable inks, paints, pigments and devices.